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The importance of fibre flocculation in flotation deinking

COST FP1005 “Fibre suspension flow modeling” 24-26 Oct. 2012, Trondheim, Norway. The importance of fibre flocculation in flotation deinking. P. Huber # , E. Zeno # , B. Fabry # , X. Rousset # , M.C. Angelier # , D. Beneventi*, T. Vazhure &. # :CTP, *: LGP2, & : Aylesford Newsprint.

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The importance of fibre flocculation in flotation deinking

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  1. COST FP1005 “Fibre suspension flow modeling” 24-26 Oct. 2012, Trondheim, Norway The importance of fibre flocculation in flotation deinking P. Huber#, E. Zeno#, B. Fabry#, X. Rousset#, M.C. Angelier#, D. Beneventi*, T. Vazhure& #:CTP, *: LGP2, &: Aylesford Newsprint

  2. concentration Tot. (%) Fibres (%) At same brightness At same cleanliness Losses (%) conc. Accepts brightness Concentration (%) flot. cell # Background • Influence of pulp concentration on flotation efficiency • (pilot flotation trials – VOITH facility) The higher the concentration, the lower the ink removal and the better the yield Britz, H., Peschl, A. (1994) , Wochenblatt für Papierfabrikation, n°15: 603-608, 1994.

  3. Background • Influence of concentration on air content • (industrial flotation trials) Air content Ink removal The higher the air content, the better the ink removal The higher the concentration, the lower the air content Dorris, G.M., Pagé, N., Gendron, S., Murray, T. & Ben, Y. (2006) Prog. Pap. Recycling, 16 (1), pp.31-40.

  4. Mechanisms • Hypothesis (Dorris et al. 2006) • High concentration •  (flocculation ?) • heterogeneous fibre suspension  chanelling • air bubbles can travel faster, coalescence and rise faster to the top of the cell • Decrease of relative residence time air/pulp • air content is reduced • ink removal is impaired

  5. Background • Influence of concentration on fibre flocculation • (laboratory trials with various pulps) • Example with TMP fibres • Same results with BKP (HW, SW, mix), DIP, etc. The higher the concentration, the higher the fibre flocculation Huber, P., Carré, B., and Petit-Conil, M. (2008). BioRes. 3(4), 1218-1227.

  6. Background • Influence of crowding factor on pulp flocculation • (flocculation varied by changing concentration and pulp mixtures HW/SW) fibre crowding determines fibre flocculation (at given turbulence) Huber P., Roux J.C., Mauret E., Belgacem N., and Pierre C. (2003), J. Pulp & Pap. Sci. 29(3):77-85.

  7. Gas hold-up (crowding) Background • Influence of crowding factor on gas hold-up • (Column bubbling of virgin pulp) The higher the fibre crowding, the lower the air content Tang, C. & Heindel, T.J. (2006) The Canadian Journal of Chemical Engineering, 84 (2), pp.198-208.

  8. Motivations & objectives • influence of pulp flocculation on flotation efficiency ? How to vary flocculation ? By adding dispersants By changing concentration • Non surface-active fibre dispersants : • Guar gum • CMC

  9. Outline • Background • Methods : flocculation sensor, gas hold-up sensor… • Results • Effect of concentration • Effect of dispersants • Mechanisms

  10. • On-line Pilot flow - loop thickness thickness = = overflow overflow 3.5 mm tank tank flow flow IMAGE meter meter ANALYSIS Floc. Floc. Floc. sensor sensor sensor CCD CCD CAMERA CAMERA FLASH FLASH TRANSPARENT TRANSPARENT CCD CCD CCD CHANNEL CHANNEL pulp pulp • Flocculation index: tank tank surface surface S S 1 m 1 m 3 3 FI i i … … D D diameter diameter i i Assessment of fibre flocculation Materials and Methods Flocculation measurement • General methods • Pulp circulation on the flocculation pilot loop • Fibre flocculation testing with the CTP FlocSens (image analysis) • Constant flow speed : equivalent shear rate = 690 s-1 (medium speed) • Flocculation sensor (+overflow) installed on Recycled fibres pilot plant, at flotation inlet

  11. surface Si … Di diameter Flocculation sensor principles • Flocculation index : • binary morphology  floc size distribution • Flocculation index:

  12. Reduced sensitivity to light diffusion • Problem : filler diffuse light (no filler) Fibres only Fibres+20% filler RMS = 0.069 FI = 2.65 mm² RMS = 0.323 FI = 2.63 mm² • Even in presence of light diffusing filler : Fibre flocculation measurement is possible (independently on filler flocculation level) Huber P., Roux J.C., Mauret E. and Carré B. (2006), APPITA Journal 59(1):37-43

  13. Flotation monitoring :Assessment of pulp aeration • Air content ≠ Air ratio • Air ratio is a mechanical parameter only • Air content is a true measurement of pulp aeration • Includes both hydraulic and physico-chemical effects • relevant parameter that affects flotation efficiency collection efficiency bubble surface area flux flotation rate constant  Sbis proportional to air content Leichtle (1998)

  14. Dorris et al. (2006) P Flotation monitoring :On-line measurement of air content • Paprican sensor • Based on pressure difference between immersed gauges • Apparent pulp density varies with air content • Installation • Installed on reject side, across the hatch, at an angle of 60° • probes installed in pre-flotation 1ry

  15. Viewing chamber CCD camera PC for image analysis Halogen light source Bubble size distribution Materials and Methods ii) Bubble size measurement • Bubble collection via a sampling pipe and visualization in a glass window • Automated bubble count using a CCD camera and image analysis software (Sherlock 7) D.Beneventi, Pilot verticell

  16. 16 g/L 8 g/L FlocculationEffect of concentration • Effect on fibre flocculation (flotation cell inlet) Fibre fraction = 56% • Pulp flocculation increases when increasing pulp concentration (8-16 g/L) • Higher crowding • More fibres interacting with each other (mainly governed by fibre concentration) •  stronger flocculation

  17. Effect of concentration: industrial trials at Aylesford • Air content • Large variations of air content over time (in 1ry) • Higher air content contributes to better ink removal efficiency • Higher air impairs flotation yield • Higher air content is caused by concentration decrease To maximise the flotation yield, work at highest possible concentration while maintaining ERIC target (But take care, it is a question of compromise: a too high consistency will induce a too high decrease in ink removal efficiency) Huber, P., Rousset, X., Zeno, E. and Vazhure, T. (2011) Ind. Eng. Chem. Res. 50(7) :4021-4028

  18. -14 to 19 % FlocculationEffect of dispersants • Selected dispersants (Guar gum and CMC) effectively de-flocculate DIP • On the fibre flocculation +1% guar +2% guar reference +1% CMC +2% CMC (Flotation cell inlet)

  19. Effect of dispersants : pilot trials • Selected dispersants (guar gum or CMC) clearly improved flotation selectivity • Better ink removal + lower losses at the same time • Increased air content thanks to pulp de-flocculation (at the fibre level) • Lower entrainment of fine elements thanks to depressing mechanism (from adsorbed dispersants) (high overall losses because of low froth height) Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010). Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329

  20. Effect of dispersants : pilot trials • Better selectivity : the link is fibre flocculation • flocculation directly influenced the pulp aeration : Gas hold-up when flocculation Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010). Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329

  21. Effect of dispersants : pilot trials • Bubble size ? Rising velocity in Newtonian fluid : Drift flux model : • Limited bubble size decrease • Not sufficient to explain increased gas hold-up • Higher drag on bubbles in de-flocculated pulp Zeno, E., Huber, P., Rousset, X., Fabry, B. and Beneventi D. (2010). Ind. Eng. Chem. Res., 2010, 49 (19), pp 9322–9329

  22. Mechanisms • Improved mechanism (this work) • Lower (fibre) concentration or add fibre dispersants • de-flocculation • homogeneous fibre suspension • higher drag on air bubbles rise slowly to the top of the cell • increase of relative residence time air/pulp • air content is increased • ink removal is improved • (limited coalescence (surfactants) limited chanelling)

  23. Conclusions • Relationship among flocculation, ink removal, turbulence and air content is not simple • Depends on hydraulic regimes in the flotation cells (turbulence pattern) • (lab cell ≠ pilot cell ≠ industrial cell ≠ various industrial flotation cells config.) • Will affect interactions between air bubbles and pulp flocs • Pulp flocculation does impact flotation efficiency • Mechanisms involved • Ink removal : • pulp de-flocculation (at fibre level) homogeneous fibre network higher  higher drag  higher air content  better ink removal • Losses : • With concentration : mechanism not clear • With dispersants : Lower entrainment of fine elements thanks to depressing mechanism (from adsorbed dispersants) • Selectivity ? • When increasing concentration : • poorer ink removal, lower losses • But little impact on selectivity • With added dispersants : • clear selectivity increase (at least at lab and pilot scale)

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